Patients with kidney failure or partial kidney failure typically undergo hemodialysis treatment in order to remove toxins and excess fluids from their blood. To do this, blood is taken from a patient through an intake needle or catheter which draws blood from an artery or vein located in a specifically accepted access location—e.g., a shunt surgically placed in an arm, thigh, subclavian and the like. The needle or catheter is connected to extracorporeal tubing that is fed to a peristaltic pump and then to a dialyzer that cleans the blood and removes excess fluid. The cleaned blood is then returned to the patient through additional extracorporeal tubing and another needle or catheter. Sometimes, a heparin drip is located in the hemodialysis loop to prevent the blood from coagulating.
As the drawn blood passes through the dialyzer, it travels in straw-like tubes within the dialyzer that serve as semi-permeable passageways for the unclean blood. Fresh dialysate solution enters the dialyzer at its downstream end. The dialysate surrounds the straw-like tubes and flows through the dialyzer in the opposite direction of the blood flowing through the tubes. Fresh dialysate collects toxins passing through the straw-like tubes by diffusion and excess fluids in the blood by ultra filtration. Dialysate containing the removed toxins and excess fluids is disposed of as waste. The red cells remain in the straw-like tubes and their volume count is unaffected by the process.
A blood monitoring system is often used during hemodialysis treatment or other treatments involving extracorporeal blood flow. One example is the CRIT-LINE® monitoring system produced by Fresenius Medical Care of Waltham, Mass. The CRIT-LINE® blood monitoring system uses optical techniques to non-invasively measure in real-time the hematocrit and the oxygen saturation level of blood flowing through the hemodialysis system. The blood monitoring system measures the blood at a sterile blood chamber attached in-line to the extracorporeal tubing, typically on the arterial side of the dialyzer.
In general, blood chambers along with the tube set and dialyzer are replaced for each patient. The blood chamber is intended for a single use. The blood chamber defines an internal blood flow cavity comprising a substantially flat viewing region and two opposing viewing lenses. LED emitters and photodetectors for the optical blood monitor are clipped into place onto the blood chamber over the lenses. Multiple wavelengths of light may be directed through the blood chamber and the patient's blood flowing through the chamber with a photodetector detecting the resulting intensity of each wavelength.
Suitable wavelengths to measure hematocrit are about 810 nm, which is substantially isobestic for red blood cells, and about 1300 nm, which is substantially isobestic for water. A ratiometric technique implemented in the CRIT-LINE® controller, substantially as disclosed in U.S. Pat. No. 5,372,136 entitled “System and Method for Non-Invasive Hematocrit Monitoring,” which issued on Dec. 13, 1999, and is incorporated herein by reference, uses this light intensity information to calculate the patient's hematocrit value in real-time. The hematocrit value, as is widely used in the art, is a percentage determined by the ratio between (1) the volume of the red blood cells in a given whole blood sample and (2) the overall volume of the blood sample.
In a clinical setting, the actual percentage change in blood volume occurring during hemodialysis can be determined, in real-time, from the change in the measured hematocrit. Thus, an optical blood monitor is able to non-invasively monitor not only the patient's hematocrit level but also the change in the patient's blood volume in real-time during a hemodialysis treatment session. The ability to monitor real-time change in blood volume helps facilitate safe, effective hemodialysis.
To monitor blood in real time, Light Emitting Diodes (LEDs) and photodetectors for them are mounted on two opposing heads of a sensor clip assembly that fit over the blood chamber. For accuracy of the system, the LEDs and the photodetectors are located in a predetermined position and orientation each time the sensor clip assembly is clipped into place over the blood chamber. The predetermined position and orientation ensures that light traveling from the LEDs to the photodetectors travels through a lens of the blood chamber.
In existing systems, the optical monitor is calibrated for the specific dimensions of the blood chamber and the specific position and orientation of the sensor clip assembly with respect to the blood chamber. For this purpose, the heads of the sensor clips are designed to mate to the blood chamber so that the LEDs and the photodetectors are at known positions and orientations with respect to one another.
While there are numerous light emitters which can be used, LEDs are often preferred due to their cost factors with their wide use in industry. In most non-medical applications, precise amplitude of the generated light is not important. For example, indicator lights showing that a device is on is only required to glow so that it is visible to the end user. Whether the amplitude (brightness) of the light changes slightly over time or temperature is of no consequence in this use. Another example where precision of amplitude is less critical is in driving fiber optic cables to propagate phone calls, video and the like over extended distance. In this application, the light source is commonly keyed on and off in patterns or time widths creating modulations where detection is by light amplitude thresholds. If the light amplitude is high enough to exceed the threshold, one digital state is registered. If not, then the opposite digital state is registered. A slight change in amplitude where the threshold is still crossed is of no consequence to the operation of the system.
However, the use of LEDs (or any light source) in blood monitoring systems such as described herein requires knowing the precise amplitude. All small variations in the amplitude are accounted for. Otherwise, errors can result in the measurements of blood parameters. For blood parameters to be repeatedly measured with acceptable accuracy, effects on the amplitude of the light that are acceptable in some applications such as telecommunications must be dealt with in blood monitoring systems.
Changes in the amplitude of the light from LEDs can be attributed to three of their physical properties.
The first property gives an effect of a “short term” amplitude shift, which affects the amplitude. During the manufacturing process of LEDs, specially formulated Silicon or Indium Gallium Arsenide compounds are melted together to form electrical junctions, making the device an LED. Impurities in the environment during the manufacturing process, although the process is performed in a clean room, can contaminate the junction. The effect is to change the amplitude that would otherwise be obtained if the junction is pure when energized with the proper current. Over time, with heat applied during normal operation of the junction, the impurities are “burned off,” causing the LED to change its output amplitude as the impurities diminish.
The second property causes a “long term” amplitude shift. This shift results from the quantum mechanics of the materials in the LEDs as they change with age. There is nothing to be done about this effect. The shift is small and requires several years for it to have an effect on the amplitude that would be noticeable in the context of applications such as blood monitoring systems.
The third property causing changes to the amplitude of the light is temperature sensitivity. The temperature at the internal LED junction directly affects the speed of the electro-chemical reaction at the junction, which in turn affects the number of electrons changing orbit. The energy released by this action is selected by the compounds used to make the LED to yield a specific wavelength of light. For example, at higher temperatures there is more electron activity in the device junction, resulting in more electron movement and, thus, greater amplitude of the light.
To address the “short term” effect on amplitude, conventional blood monitoring systems often rely on a base calibration model to yield a known, quantified amplitude for an LED. A “burn-in” process deliberately raises the LED junction temperatures using high current (but not high enough to harm the device's junction) to rapidly dissipate any manufacturing impurities in the junction and bring “short term” stability to the LED.
To address the “long term” effect on amplitude, the variation is slow enough that conventional blood monitoring systems are usually returned for service or for other reasons prior to this effect become noticeable in the context of the system's performance.
The temperature effect on the amplitude of the light from LEDS is addressed in many conventional blood monitoring systems by employing a compensation model that relies on a relationship between temperature and amplitude variations established through measurements. The blood monitoring system uses a thermistor sensor mounted in close proximity to the LEDs to measure the average temperature of the LEDs. The temperature signal from the thermistor is provided to the compensation model that compensates for variations in the amplitude of the light from the LEDs as a function of their temperatures. The compensation model includes empirical data collected for each LED. The compensation model of each blood monitor system is calibrated for the temperature profile of its LEDs. Thus, each monitor channel has a temperature calibration model based on the temperature profile for the LED for which it provides compensation. Moreover, the average temperature of all LEDs in a system is typically used for the compensation, causing errors in measurement in the event of a single LED fluctuation. Also, measuring light output by sensing the temperature profiles of the LEDs and then mapping the actual temperatures to light amplitude can become inaccurate as the LEDs age (the “long term” effect).